An ice shelf is a large platform of glacial ice floating on the ocean, fed by one or multiple tributary glaciers. Ice shelves form along coastlines where the ice thickness is insufficient to displace the more dense surrounding ocean water. The boundary between the ice shelf (floating) and grounded ice (resting on bedrock or sediment) is referred to as the grounding line; the boundary between the ice shelf and the open ocean (often covered by sea ice) is the ice front or Ice calving front.
Ice shelves are found in Antarctica and the Arctic (Greenland, Northern Canada, and the Russian Arctic), and can range in thickness from about . The world's largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica.
The movement of ice shelves is principally driven by gravity-induced pressure from the grounded ice.[ Glossary of Glacier Terms, Ellin Beltz, 2006. Retrieved July 2009.][Essentials of Geology, 3rd edition, Stephen Marshak] Snow accumulation on the upper surface and melting from the lower surface are also important to the mass balance of an ice shelf. Ice may also accrete onto the underside of the shelf.
The effects of climate change are visible in the changes to the cryosphere, such as reduction in sea ice and ice sheets, and disruption of ice shelves. In the last several decades, glaciologists have observed consistent decreases in ice shelf extent through melt, Ice calving, and complete disintegration of some shelves. Well studied examples include disruptions of the Thwaites Ice Shelf, Larsen Ice Shelf, Filchner–Ronne Ice Shelf (all three in the Antarctic) and the disruption of the Ellesmere Ice Shelf in the Arctic.
Definition
An ice shelf is "a floating slab of ice originating from land of considerable thickness extending from the coast (usually of great horizontal extent with a very gently sloping surface), resulting from the flow of
Ice sheet, initially formed by the accumulation of snow, and often filling embayments in the coastline of an ice sheet."
[IPCC, 2021: Annex VII: Glossary Matthews,. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Masson-Delmotte,. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.]
In contrast, sea ice is formed on water, is much thinner (typically less than ), and forms throughout the Arctic Ocean. It is also found in the Southern Ocean around the continent of Antarctica.
The term captured ice shelf has been used for the ice over a subglacial lake, such as Lake Vostok.
Properties
Ice shelves are thick plates of ice, formed continuously by glaciers, that float atop an ocean. The shelves act as "brakes" for the glaciers. These shelves serve another important purpose—"they moderate the amount of melting that occurs on the glaciers' surfaces. Once their ice shelves are removed, the glaciers increase in speed due to meltwater percolation and/or a reduction of braking forces, and they may begin to dump more ice into the ocean than they gather as snow in their catchments. Glacier ice speed increases are already observed in Peninsula areas where ice shelves disintegrated in prior years."
Height
The density contrast between glacial ice and liquid water means that at least of the floating ice is above the ocean surface, depending on how much pressurized air is contained in the bubbles within the glacial ice, stemming from compressed snow. The formula for the denominators above is
,
Seawater is about 1028 kg/m
3 and that of glacial ice from about 850 kg/m
3 to well below 920 kg/m
3, the limit for very cold ice without bubbles.
The height of the shelf above the sea can be even larger, if there is much less dense
firn and snow above the glacier ice.
By country or region
Antarctica
A large portion of the Antarctic coastline has ice shelves attached.
Their aggregate area is over .
It has been found that of all the ice shelves on Earth, nearly all of them are in Antarctica.[IPCC, 2021: Annex VII: Glossary Matthews,. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Masson-Delmotte,. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.]
In steady state, about half of Antarctica's ice shelf mass is lost to basal melt and half is lost to ice calving, but the relative importance of each process varies significantly between ice shelves.
Ross Ice Shelf
Filchner–Ronne Ice Shelf
Arctic
Canada
All Canadian ice shelves are attached to
Ellesmere Island and lie north of 82°N. Ice shelves that are still in existence are the Alfred Ernest Ice Shelf, Ward Hunt Ice Shelf, Milne Ice Shelf and Smith Ice Shelf. The M'Clintock Ice Shelf broke up from 1963 to 1966; the Ayles Ice Shelf broke up in 2005; and the Markham Ice Shelf broke up in 2008. The remaining ice shelves have also lost a significant amount of their area over time, with the Milne Ice Shelf being the last to be affected, with it breaking off in August 2020.
Russia
The Matusevich Ice Shelf was a ice shelf located in
Severnaya Zemlya being fed by some of the largest ice caps on October Revolution Island, the Karpinsky Ice Cap to the south and the
Rusanov Glacier to the north.
[Mark Nuttall, Encyclopedia of the Arctic, p. 1887] In 2012 it ceased to exist.
Disruption due to climate change
In the last several decades, glaciologists have observed consistent decreases in ice shelf extent through melt,
Ice calving, and complete disintegration of some shelves. Well studied examples include disruptions of the Thwaites Ice Shelf, Larsen Ice Shelf, Filchner–Ronne Ice Shelf (all three in the Antarctic) and the disruption of the Ellesmere Ice Shelf in the Arctic.
The effects of climate change are visible in the changes to the cryosphere, such as reduction in sea ice and ice sheets, and disruption of ice shelves.
Disruption of Thwaites Ice Shelf
Disruption of Larsen Ice Shelf
Two sections of Antarctica's Larsen Ice Shelf broke apart into hundreds of unusually small fragments (hundreds of meters wide or less) in 1995 and 2002, Larsen C calved a huge ice island in 2017.
Disruption of Larsen B Ice Shelf
Disruption of Filchner–Ronne Ice Shelf
Other ice shelves in Antarctica
-
Wordie Ice Shelf has gone from an area of in 1950 to in 2000.
-
Prince Gustav Ice Shelf has gone from an area of to in 2008.
-
The Ross Ice Shelf is the largest ice shelf of Antarctica (an area of roughly and about across: about the size of France).
-
Wilkins Ice Shelf is another ice shelf that has suffered substantial retreat. The ice shelf had an area of in 1998 when was lost that year.
Disruption of Ellesmere Ice Shelf (Arctic)
The Ellesmere ice shelf was reduced by 90% in the twentieth century, leaving the separate Alfred Ernest, Ayles, Milne, Ward Hunt, and Markham ice shelves. A 1986 survey of Canadian ice shelves found that 48 km
2 (3.3 cubic kilometres) of ice calved from the Milne and Ayles ice shelves between 1959 and 1974.
[Jeffries, Martin O. Ice Island Calvings and Ice Shelf Changes, Milne Ice Shelf and Ayles Ice Shelf, Ellesmere Island, N.W.T. . Arctic 39 (1) (March 1986)] The Ayles Ice Shelf calved entirely on August 13, 2005. The Ward Hunt Ice Shelf, the largest remaining section of thick (>)
Fast ice along the northern coastline of Ellesmere Island, lost of ice in a massive calving in 1961–1962.
[Hattersley-Smith, G. The Ward Hunt Ice Shelf: recent changes of the ice front. Journal of Glaciology 4:415–424. 1963.] It further decreased by 27% in thickness () between 1967 and 1999.
[Vincent, W.F., J.A.E. Gibson, M.O. Jeffries. Ice-shelf collapse, climate change, and habitat loss in the Canadian high Arctic. Polar Record 37 (201): 133–142 (2001)] In the summer of 2002, the Ward Ice Shelf experienced another major breakup,
and other instances of note happened in 2008 and 2010 as well.
The last remnant to remain mostly intact, the Milne Ice Shelf, also ultimately experienced a major breakup at the end of July 2020, losing over 40% of its area.
See also
External links
